The invention relates to a switching arrangement for pulse code modulation (PCM) time division multiplex switching systems having switching units assigned individually to the PCM time division multiplex primary trunks which have for each direction of transmission a frame storage for storing information, crosspoint elements in direct communication with the frame storages and address memories for activating the frame storages and the crosspoint elements and which, together with the other switching units, form a TDM switching network having a time-space-space-time switching configuration.
In a similar switching arrangement of known construction (cf. West German patent application No. P 21 08 745. 6-31) the frame storage for information for the incoming direction of transmission can be connected in a switching unit to one wire pair of the PCM time division multiplex trunk, and the frame storage for information for the outgoing direction of transmission is connected to the other wire pair of the PCM time division trunk. The read-in operation for the PCM time division multiplex trunk to the frame storage for the incoming direction of transmission and the read-out operation from the frame storage for information for the outgoing direction of transmission recur in successive cycles. A readout from the frame storage for information for the incoming direction of transmission can only take place toward a space-division section of the switching network. Accordingly, a read-in to the frame storage for the outgoing direction of transmission is only possible from this space-division switching network. Both the read-in to the space-division switching network portion and the readout from there occur randomly. In the course of one and the same call the same switching time position is used both for the random readout and for the random read-in operations. For this reason, a single address memory is sufficient for activating a frame storage for the incoming direction of transmission and a frame storage for the outgoing direction of transmission.
If a return switching operation to the same PCM primary trunk is to be effected in such a switching arrangement, i.e., from a time slot formed on the incoming pair of line wires to one formed on the outgoing line pair of the same trunk, a further switching unit is required. That switching unit is not connected to a PCM trunk, but its input and output are connected together. This switching unit is in communication with the switching units in a group of switching units in the same way via the space-division section of the switching network as the switching units connected to PCM trunks are in communication with one another.
In the switching arrangement of known construction an extremely unbalanced traffic situation may cause links in a space-division section of the switching network that connect together prespecified switching units to become busy resulting in traffic losses. This situation may likewise occur if, for reasons of safety, the switching units in question are provided in pairs and designed for accepting half the traffic load and if one of these parallel switching matrices fails.
Overall, the switching arrangement of known construction was not particularly suited for production by large-scale integration (LSI) techniques.
Therefore, it is an object of the invention to provide a switching arrangement of the type described hereinabove which permits easy return switching to the same PCM primary trunk, works substantially free of losses and without additional complexity and expense and is suitable for production by large-scale integration techniques.